Frequency modulated radio-type distance measuring device



M. V. KIEBERT, JR FREQ NCY MODULATED RADIO-TYPE DISTANCE MEASURING DEVICE March 6, 1951 Filed May 26, 1945 www yjaeemf.

35?/ MHALCMWU Patented Mar. 6, 1951 UNITED STATES PATENT OFFICE FREQUENCY MODULATED RADIO-TYPE DISTANCE MEASURING DEVICE (Granted under the act of March 3, 1883, as

` amended April 30, 1928; 370 0. G. 757) This invention relates to apparatus for determining the distance between two objects such as the altitude of an aircraft from the ground and in particular to such apparatus of the radio type in which the distance is measured by transmitting a frequency modulated (FM) electromagnetic Wave from the aircraft to the ground, receiving the reflected wave from the ground and determining the time interval between transmission and reception. Since the velocity of electromagnetic wave energy is a constant, the distance between aircraft and ground will obviously be a function of the time interval.

A general object of this invention is to provide an improved construction for an FM radio-type distance measuring device in which only a single scale indicator need be used to indicate distance accurately over a wide range.

A more specific object of this invention is to provide a frequency modulated (FM) radio-type distance measuring device in which the bandwidth of the frequency modulation imposed upon the carrier wave is compressed automatically as the distance is increased, as distinguished from present apparatus which utilizes a plurality of fixed FM bands, and in which the bands are switched from one to the other dependent upon distance. Thus, those components of the apparatus which follow the detector stage in my improved construction do not have to be changed to accommodate band switching; hence, a singlel scale distance indicator can be used to indicate distance accurately over a very wide range. This is accordingly a decided improvement over apparatus of present design in which a different scale must be provided in the distance indicating instrument for each' of the bandwidths of frequency modulation used. That is to say, for example, if there are two bandwidths of frequency modulation employed to cover the complete distance range, the indicating instrument must hav two scales of distance graduations, etc.

Still another object is to provide a distance measuring device of theclass described in which the bandwidth of frequency modulation imposed, upon the high frequency carrier wave is varied automatically as a function of the beat frequency between the transmitted and reected waves.

. A still further object is to provide a distance measuring device of the class described in which the bandwidth of the frequency modulation imposed upon the carrierwave is varied by changing the amplitude of the modulator unit assotion of the beat frequency output of the detector stage of the apparatus.

In one type of apparatus of current design for measuring altitude, there is emitted, in a downward direction from a transmitter antenna, a radio-frequency carrier wave which is frequency modulated at a rate of cycles per second between the approximate limits of 420 to 460 megacycles on the low range (0 to 400 feet) and 443 to 447 megacycles on the high range (400 to 4000 feet). The earths surface reflects some of this carrier, and the reflected signal is received on a separate receiver antenna. During the time interval required for the signal to travel to earth and return to the aircraft, the transmitter frequency has, of course, changed. The combination of the reflected signal with a signal received directly from the transmitter results, by process of detection, in an audio-frequency signal, the average frequency of which is' proportional to the altitude of the aircraft above ground. After passingthrough frequency counter circuits, the detected signal is used to operate a meter which is calibrated directly in feet of altitude.

If symmetrical sawtooth frequency modulation be assumed, it can be shown that when the instantaneous frequency of the transmitted signal is combined with the reflected signal, the beat frequency difference fd between the two signals may be computed from the equation fd=2fmBX105 2h cycles per second where fm=modulator sweep frequency, in cycles per second;

B=bandwidth` in megacycles (FMX-Finis) h=altitude, in feet (one-half the distance traveled by the radio wave); and

C=velocity of propagation=984 l06 ft. per

second.

Thus with a modulator frequency fm of 120 cycles, and with bandwidths of 460-420 or 40 megacycles 0n the low range, and 447-443 or 4 megacycles on the high range, the beat frequency difference per foot of altitude may be evaluated ciated with the high frequency oscillator componentuof the transmitter apparatus as a func-.

at approximately 19 cycles per second for the low range, and 1.9 cycles per second for the high range. At the maximum altitude of 400 feet on the low range and also 4000 feet on the high range, the beat frequency difference is approximately 7600 cycles per second.

If sinusoidal frequency modulation is assumed,

the analysis differs from that given above for v3 sawtooth or linear modulation in a few details. However, under normal operating conditions, the average beat frequency output for a given altitude will be equal to the constant beat frequency difference obtained when sawtooth modulation is used.

Up tothe present time, it has been necessary to utilize two bandwidths on the carrier wave in order to produce an instrument which can be used over an acceptable range in altitude such as from zero to 4000 feet. The reason for this is that if accuracy is to be maintained, the detector `must have a linear response over its entire range 'of operation. Furthermore, the signal to noise .-ratio must remain high throughout the beat frequency range so that the beat frequency counters will function accurately. Thusif fa fixed bandwidth of 40 megacycles and a modul-ationifrequency of 120 cycles per second'fbe assumed, it is evident from the equation given above, that atan altitude of 4000 feet, the beat frequency Vdifference would rise to '76,000 cycles; l'and lat 'an extremely low altitude of, say 50 feet, the be'a't frequency difference would be only 95"() cycles. The dilculties of designing such a wide band audio frequency detector and amplifier circuit which would retain va linear characteristic over vso wide an Vaudio range are obvious.` l By use of a much lower bandwidth, the eX- tremes of the beat frequency would be within such limits that an audio frequency detector and amplifier -unit would retain a linear output characteristic. However, with apparatus of present design, the beat frequency difference output of the detector is amplified and passed through a square wave limiter stage before reaching =the counter circuits in order `to increase the accuracy ofthe counting function. The frequency counter circuits thus measure the average frequency of the square 'wave limiter output 'which vmay not be the exact average of the instantaneous beat frequency difference. The frequency iof the limiter output actually varies with respect to .the altitude in definite steps (multiples of the modulator frequency). n

It can be shown Ythat under 4flight 'conditions wherein the rate of change lof altitude 'is zvery low, these frequencystepsfmay be translat'edinto steps of indicated 'altitude thus resulting Vin an inaccuracy which has become known as the fixed error. The actual magnitude of 4this error is equal to (E) Y B where B is the bandwidth in megacycles.

Thus were a relatively low and fixed bandwidth such as 4 megacycles vto be used, the i"fxed^error would be equal to 'approximately 6l feet. Y The vindicating instrument `could thus not be relied raudio frequency components of the apparatus lto derive an output variable with .fd from which altitudes in the lower rangecould .be measured with any acceptable degree of accuracy.

So far, the answer `to the problem has rbeen to use the two-bandwidth system of Ythe type described, a rather wide band'being used for the lower altituderange and a rather narrow onebe- 4 ing used for the upper altitude range. Two range scales in the distance indicator are accordingly required.

While the dual-range altitude indicating system described is, in itself, satisfactory, it is obvieusly imperative that the rpilo't reme'mber at all times which Vrange is in use. With the many other duties imposed upon the pilot, it is obvi- =ously quite possible for him to forget the particular range .scale in use and often with fatal results.

The object of this invention, as previously improvement upon the .presently designed apparatus to the end that a single scale instrument of wide range can be used, and :it will give accurate results over its entire range of operation. Y Y

I n J'the drawings,

Figure 1 is V'a block .diagram which illustrates a preferred embodiment of the invention4 as applied to radio altimeter apparatus; and

Figure II shows curves `which explain diagrammatical'ly the manner in which frequency -modulation altir'netfers operate.

Referring now to I, a lradio frequency signal is generated vin transmitter unit l0. This signal is 4frequency modulated :by a device Il whichinclude's VVa permanentniaignetic field (not shown) 'anda Vmoving "'coil |22 Vdriving .a .metal diaphragm "|53 which acts as :the moving :plate of a frequency modulator condenser. Diaphragm I3 is actuated'by a nearly sinusoidal voltage supplied to the moving coil l2 b'yaniaudio frequency modulation generator '$14 at la :selected repetition rate, and moves relative Ato a 'pair of `fixed condenser plates TE, :16. 'The amplitude `of the -youtput lvoltage 'from modulation frgenerator I4 determines the maximum Email and Aminimum Fmm limits ro'f V'modulation of the R. F. A'carrier fsignal generated fin transmitter ill), :and ithe period fof this voltage lcleterr'nines 'the repetition `rate or modulation "cycle Vif lcarrier ,frequency modulation.

An electronic fattenuator :unit Il Vfis inserted in thefline :betweenimodulation generator :I 4 and the frequency l:trodulator device il l, and its function will b'eexplainedzf-in more .detail hereinafter.

The .frequency modulated signal -output from transmitter Hl .is :fed to a half-wave -di-pole f antenna 1t8 'carried by the aircraft and radiated :therefrom iin .fa downward `V'direction along` path 'I 9 tofthefeartwssurface 20. .IAlportionpf this .radiated signal is 'reflectede'backealong'path 21 and returns to Athe aircraft, being :received f on antenna `22,2wliioh may ralso befafhalf-'wave dipole. The relation 'between the :transmitted vand reflected the 'outputifrom transmitt'er al 0 rhas :changed fduring Ythe .time

required-furthe wave-energy 'to traverfrom the aircraft tothe earth "andfbe `vreflected back "to ithe aircraft. vThe direct 'and `'reflected "signals are then heterodyn'ed in "detector 23 to 'produce an output voltage V,i which varies as `the beat Ifrequency vdifference fd" between`-these two signals.

The iitplit from "detector "23 *is *then-amplified `in a multi-stage amplifier unit 25 and fed to a indicator counter 21. Counter 21 develops a positive D. C. voltage which is approximately proportional to the frequency of the applied square wave 1 limiter signal.

The output from counter 21 is fed to an inlt'licator amplier stage 28, the function ofwhich is to supply a current proportional to the output from counter 21 to an altitude indicator meter 29.

Thus meter 29 may be calibrated in feet of altitude, since it has already been shown from the previously included equation that altitude is proportional to the rinstantaneous beat frequency difference fd between the frequency of the transmitted signal and the frequency of the reflected signal. Y

Assuming that the bandwidth B of the frequency modulated signal output from transmitter I0 remains fixed, the FM radio altimeter apparatus so far described excepting attenuator unit |1 is conventional. A somewhat similar system, from a basic point of view, is disclosed in U. S. Patent No. 2,206,903, issued July 9, 1940, to R. F. Lane et a1.

However, as previously explained in the opening part of this specication, accurate indication of altitude over a comparatively wide range, say i from 0 to 4000 feet, with a fixed bandwidth of frequency modulation of the carrier wave has not been possible with equipment of present de- 3 sign, and the solution so far has been to provide for two operating bandwidthsa wide band, for example'fl 'megacycles for the lower range Iof altitude (0-400 feet), and a narrow band, for example 4 megacycles, for the higher range of altitude (400-4000 feet). The two bands are used selectively, and the pilot manually switches from one to the other depending upon the particular range of altitude at which he is flying. Of course, two different scales would then have to be used on indicator 29, and the pilot would always have to remember which scale was in use.

The object of this invention which is to so improve the presently designed apparatus that a single scale altitude indicator meter can be used, and which will measure altitude accurately over a very broad range, is attained by automatically compressing or reducing the bandwidth B of the frequency modulation of the high frequency carrier wave as a function of the increase in beat frequency difference fo between the transmitted and reflected signals,

In the preferred embodiment of the invention, I continually adjust the bandwidth B by taking an output from the indicator counter 21 Whieh, as previously explained, is proportional to the beat frequency difference fd and utilize the same to control the previously mentioned attenuator I1.

Attenuator l1 includes a pair of tubes 3| and Y32, The output from modulation generator I4 is applied through transformer coupling 33 to the grid Sla of tube 3|. The anode 3Ib of this tube `feeds moving coil I2 of the frequency modulation control unit A resistor 34 is connected in circuit with the cathode 3|c of tube 3| and across this resistor, the anode 32b and cathode 32e of tube 32 are connected. The grid 32a of of tube 32 is connected over line 35 to the output of indicator counter 21.

The arrangement is such that as the beat frequency dilerence fd increases, the positive voltage at the output of counter 21 likewise increases.

-6 This increase in voltage,v which is then applied over line 35 to the grid 32a of tube 32, changes the voltage drop across resistor 34 and hence the bias on grid 3|a of tube 3| in such a manner as to decrease the current flow in the anode-catliode circuit of this latter tube. Current iiow through coil I2 is likewise decreased resulting in a decrease in amplitude of motion of the movable condenser plate |3. Since the bandwidth B of the frequency modulation applied to the carrier wave generated in transmitter 0 varies with the amplitude of` motion of condenser plate |3, a

.decrease in this amplitude produces a decrease in bandwidth B. Conversely, an increase in current iiow in the anode-cathode circuit of tube :3| caused by a decrease in beat frequency difference fd will effect an increase in bandwidth B. The overall effect is that as the altitude is increased, the beat frequency difference fd will increase, notwithstanding the accompanying compression of the bandwidth B. However, because of bandwidth compression, fd increases much more slowly than in apparatus of present design and may be kept within such limits that detector 23 and the audio components which follow it in circuit will exhibit a linear characteristic throughout their entire operating ranges. Thus only one scale need be provided on the distance indicator 29, and since the output of the aforementioned components remain linear, the indicator will likewise register distance accurately throughout its entire operating range.

Furthermore, the fixed error in the altimeter apparatus will no longer be a constant quantity but will vary with the altitude to the desirable end that at the lower altitudes, the magnitude of the error will decrease to an inconsequential amount.

In conclusion, it is to be understood that while the embodiment described herein is preferred, various changes in the construction and arrangement of parts may be made without departing from the spirit and scope of the invention as delined in the appended claims. For example, the electronic attenuator I1 may be replaced with other types, the particular one utilized being determined largely by the particular design of the device used for frequency modulating the carrier wave, of which there are many.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

1. In an apparatus for measuring the distance between an object and a reflecting surface having a transmitter on said object for transmitting a high frequency radio carrier wave towards and for reiiection from said surface, modulator means for cyclically varying the frequency of said carrier wave, and detector means combining the wave direct from said transmitter with the reected wave to produce a beat frequency wave: a system for gradually compressing the band over which said carrier wave is modulated as the frequency of said beat frequency wave increases, said system comprising a control tube, an amplifier tube for providing an input signal to the modulator means, means coupled between the detector and said control tube for producing a potential proportional to the frequency of the beat frequency wave and for applying said potential to the grid of said control tube, the plate of which is connected to the cathode of said sistaince .of said arnpliiier tube -so amarte arriplier tube for varying the 'plate 4.dsfnamic re- Y fas to eiectively change the current in -th'e modulator 4-rrieans in 'accordance with the Signal on the grid of said Control tube, While maintaining variation ol the plate current of said amplifier tube lin accordance with the signal on "the grid of said amplier tube.

2. ln an apparatus for measuring the distance between an obect and a reflecting surface llaving a transmitter, 'a medulator for cyclically varying the frequency of the transmitted Wave, and a :detector for combining the transmittedand re- 'ected waves to 'produce a -beat `frequency Wave: a system for gradually Acompres-sing the band over which trie transmitted `wave 'isimodulated as the beat frequency increases, said system eem.- 'prising an amplifier tube Vfor supplying an input signal tothe mo dulatoi, "a control tube having a grid, and means 'coupled between said detector and `said V'control tube for producing a potential substantially proportional to the frequency of the beat frequency AWave and for applying said potential to the 'grid of said control tube, the plate of 'said control tube rbeing connected to the cathode of said Aamplifier tube so as to effectively ch'ange the current `applied to the l'modulator in accordance with said potential, while maintaining variation of the plate current of said ampliner tube in accordance with the signal applied `to the grid thereof.

3. In an apparatus for measuring the distance between an object l'and a reflecting surface having 'a transmitter, a modulator 'for cyclically varying the frequency 'of the transmitted wave, a

'8 detector for combining @the transmitted :and re= ileeted waves to produce la beat frequency wave. andgjmeans for deriving a direct current potential substannually proportional to the frequency of the beatlfrequency Wave: asystem for gradually com.- pre'sslng the band :over which the transmitted Wave is modulated as the `beat -ffreque'ncy increases, said system comprising a tube for :supplying 1an input signal 'to the modulator, "a control tube having a grid, and means coupled between the deriving means and ,said control 'tube 'for applying 'the direct .current potential to said grid, 4the plate of `said control tube being connected to the Y'cathode of the 'first-mentioned tube fer vvarying the plate dynamic resistance thereof so `as'fto effectively change the cur-rent :applied tothe 'modulator in accordance with the `potential, While-maintaiini-n'g variation y:of the plate current of said first-mentioned tube in. accordance ywith the signal applied to the grid thereof. Y V. KIEBERI,JR.

,REFERENCES 'CITED 'The following references are of record in the nie of this patenti:

` UNITED STATES PATENTS Y Number Artzt June `8, 1943 

